The music event's promoter Live Nation has explained that hundreds of festival-goers were denied entry after turning up with counterfeit passes, which they had bought from eBay and Gumtree.
"This is outrageous and has to stop," a Live Nation spokesperson said. "We urge fans to only use the official ticket outlets at all times."
The promoter said fraudulent tickets had been created by third-parties, who aimed to prey on music fans desperate for tickets after the event sold out, reports PA.
Ticket fraud victim Naomi Clark told Digital Spy: "I bought four tickets for the Saturday and Sunday from a girl on Gumtree.
"She was selling them at a very reasonable price, but she said that she and her and family were going on holiday so they couldn't go anymore.
"I met up with her at Kings Cross and handed over the money, so pleased that I would be going to see Rihanna with my little siblings!
"However, on the Saturday, as we approached the entrance, we were told that the e-tickets we had weren't valid. I assumed they had been cancelled and reissued, as we had booking confirmation for them.
"I was so disgusted as we thought we were saving money by buying these reduced tickets, but instead ended up losing a lot more."
Music fans who bought the fake tickets have been urged to contact the National Fraud Intelligence Bureau through its official website.
Rihanna, Drake and Deadmau5 headlined the three-day event, which also saw sets from Nicki Minaj, Jessie J and Calvin Harris.